In this paper, we propose a social-based privacy- preserving packet forwarding protocol, called SPRING, for vehicular delay tolerant networks (DTNs). With SPRING, Roadside Units (RSUs) deployed along the roadside can assist in packet forwarding to achieve highly reliable transmissions. In specific, we first heuristically define how to evaluate each traffic intersection's social degree in a vehicular DTN. Based on the social degree information, we then strategically place RSUs at some high-social intersections. As a result, these RSUs can provide tremendous assistance in temporarily storing packets and helping packet forwarding to achieve high delivery ratio. Performance evaluations via extensive simulations demonstrate the SPRING's efficiency. In addition, detailed security analyses show that the proposed SPRING can achieve conditional privacy preservation and resist most attacks existing in vehicular DTNs.